142,009 research outputs found
Toward a theory of the integer quantum Hall transition: continuum limit of the Chalker-Coddington model
An N-channel generalization of the network model of Chalker and Coddington is
considered. The model for N = 1 is known to describe the critical behavior at
the plateau transition in systems exhibiting the integer quantum Hall effect.
Using a recently discovered equality of integrals, the network model is
transformed into a lattice field theory defined over Efetov's sigma model space
with unitary symmetry. The transformation is exact for all N, no saddle-point
approximation is made, and no massive modes have to be eliminated. The naive
continuum limit of the lattice theory is shown to be a supersymmetric version
of Pruisken's nonlinear sigma model with couplings sigma_xx = sigma_xy = N/2 at
the symmetric point. It follows that the model for N = 2, which describes a
spin degenerate Landau level and the random flux problem, is noncritical. On
the basis of symmetry considerations and inspection of the Hamiltonian limit, a
modified network model is formulated, which still lies in the quantum Hall
universality class. The prospects for deformation to a Yang-Baxter integrable
vertex model are briefly discussed.Comment: 25 pages, REVTEX, calculation of sigma_xx correcte
An overview of the focus of the International Gap Junction Conference 2017 and Future Perspectives
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Quantum phase transition in capacitively coupled double quantum dots
We investigate two equivalent, capacitively coupled semiconducting quantum
dots, each coupled to its own lead, in a regime where there are two electrons
on the double dot. With increasing interdot coupling a rich range of behavior
is uncovered: first a crossover from spin- to charge-Kondo physics, via an
intermediate SU(4) state with entangled spin and charge degrees of freedom;
followed by a quantum phase transition of Kosterlitz-Thouless type to a
non-Fermi liquid `charge-ordered' phase with finite residual entropy and
anomalous transport properties. Physical arguments and numerical
renormalization group methods are employed to obtain a detailed understanding
of the problem.Comment: 4 pages, 3 figure
Color identification testing device
Testing device, which determines ability of a technician to identify color-coded electric wires, is superior to standard color blindness tests. It tests speed of wire selection, detects partial color blindness, allows rapid testing, and may be administered by a color blind person
Constraints to the Masses of Brown Dwarf Candidates from the Lithium Test
We present intermediate dispersion (0.7-2.2 \AA ~pix) optical
spectroscopic observations aimed at applying the ``Lithium Test'' to a sample
of ten brown dwarf candidates located in the general field, two in young open
clusters, and two in close binaries. We find evidence for strong Li depletion
in all of them, and thus infer lower mass limits of 0.065~M, depending
only slightly (0.005~M) on the interior models. None of the field
brown dwarf candidates in our sample appears to be a very young (age
~10~yr) substellar object. For one of the faintest proper motion
Pleiades members known (V=20.7) the Li test implies a mass greater than
0.08~M, and therefore it is not a brown dwarf. From our spectra
we estimate spectral types for some objects and present measurements of Halpha
emission strengths and radial velocities. Finally, we compare the positions in
the H-R diagram of our sample of brown dwarf candidates with the theoretical
region where Li is expected to be preserved (Substellar Lithium Region). We
find that certain combinations of temperature calibrations and evolutionary
tracks are consistent with the constraints imposed by the observed Li depletion
in brown dwarf candidates, while others are not.Comment: 20 pp.; 4 figs, available under request; plain LaTeX, ApJ in press,
OACatania-94-00
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